1. Field of the Invention
The present invention relates to the field of acceleration sensors.
2. Description of the Related Art
Nowadays, acceleration sensors are increasingly used in many areas, and, in particular, in the field of automotive engineering. For example, acceleration sensors are used in motor vehicles as sensitive elements for applications such as airbags, knocking sensors or an intelligent vehicle chassis.
In state of the art, acceleration sensors are known to be formed of a spring and a suspended mass. A prior art acceleration sensor is described, for example, in Frank Goodenoogh, “Airbags boom when IC accelerometer sees 50 G”, Electronic Design 45, 8 Aug. 1991. The acceleration sensor comprises a test mass which is connected to a substrate by means of springs, which, for example, are each arranged at opposing sides of a first and a second end of the test mass. When an acceleration or delay occurs, the test mass will be deflected, wherein a detection of the deflection may be effected on a capacitive basis or by means of the piezo effect.
In capacitive detection, the mass is typically implemented as a panel, which, together with an electrode opposing the panel, forms a capacitor. If the mass is deflected upon accelerating or delaying, the capacity of the capacitor will change such that the measured capacity represents a measure for acceleration. If resistors are applied to the springs, the deflection of the mass and thus the acceleration may also be measured by means of the piezo effect. Furthermore, in capacitive detection, the mass may also be implemented as a comb, the teeth of which are movably arranged between the electrodes. The arrangement of the teeth in the electrodes enables a sensitive detection, since a high capacity change is caused between the teeth and the electrodes when the mass is deflected.
Nowadays, the manufacture of the above-described acceleration sensor is typically performed by means of bulk-micromachining or surface micromachining.
Structures and manufacturing methods of prior art acceleration sensors are for example explained in the above-mentioned document.
Another type of sensitive elements adapted to be generated by micromechanical methods which are frequently used nowadays is represented by a pressure sensor. Nowadays, prior art pressure sensors are used, for example, in the field of automotive engineering as side-airbag sensors or for engine control.
The prior art pressure sensors comprise a diaphragm clamped on all sides and which bends upon pressure load. The diaphragm and the counter-structure typically define a hermetically sealed closed cavity comprising a vacuum or a very low pressure. A bending of the diaphragm results depending on the internal pressure of the cavity and the outer ambient atmospheric pressure applied to the diaphragm. Since the diaphragm is hermetically sealed as against the environment, the bending of the diaphragm represents a measure for the external pressure applied to the diaphragm. As in the case of prior art acceleration sensor, the bending may be detected on a capacitive basis or by means of the piezo effect.
A prior art pressure sensor, in which an evaluation circuit is monolithically integrated by means of silicon planar technology, is for example described in H.-J. Timme et al. “Monolithic Pressure Sensor Microsystems”, Sensor 97 Int. Congress, Nuremburg, 13-15, May 1997.
In contrast to the above-described prior art acceleration sensors, in which, for increasing the sensitivity, the test mass should be as great as possible, it is considered advantageous to provide a possibly thin and low-weight diaphragm in the pressure sensor. This enables a high degree of flexibility of the diaphragm and further a low degree of influence if the pressure sensor is subjected to acceleration.
Further, for avoiding a contamination as a result of harmful substances or corrosion, a pressure sensor may be protected by means of a flowable filler, for example a gel. A sensor device, in which such a protective means is provided, is described, for example, in the document DE 196 26 086 A1.
A significant advantage of pressure sensors consists in that the same may be manufactured in an inexpensive and simple manner. In contrast to the acceleration sensors, in which extensive and precise etching processes for generating the thin springs are required, pressure sensors merely necessitate the generation of a diaphragm and a counter-structure.
In surface micromachining, generating the diaphragm and the counter-structure may be effected such that the sacrificial layer, which is arranged between the substrate and the diaphragm, is isotropically etched by means of etching holes which were generated in the diaphragm such that a cavity is formed in the sacrificial layer. Thereupon, the etching holes in the diaphragm will be closed. The sensitivity of the pressure sensor may be determined by providing trenches on an edge of the diaphragm. Further, a tab may be formed on the diaphragm which comprises oxide material, such as e.g. BPSG material (BPSG=phosphor silicon glass), plasma oxide or plasma nitride.